it is already known to anchor a tooth prosthesis permanently in the jaw with the aid of securing elements in the form of titanium screws, so-called fixtures, which are implanted in the jawbone. The screws are anchored in holes in the bone so that the upper part of the screw is situated to level with or just below the upper surface of the jawbone. The screw is then covered over with a flap of mucous membrane and is left unloaded for a rest period of 3-6 months in order for the bone to fix firmly to the implanted screw and form a unit therewith. After the rest period, the screw is exposed, and a spacing member, also preferably of titanium, is screwed into place, after which the tooth prosthesis is anchored on the spacing member. The tooth prosthesis must in this case be exactly matched to the appearance of the jaw with the titanium fixtures implanted.
In order to meet the individual requirements of different patients, there are a number of different types of spacing members on the market. The BRANEMARK SYSTEM, which is marketed by Nobelpharma AB, includes a number of standard spacers which consist of spacing cylinders of different lengths, from 3 to 10 mm, and separate spacing screws for securing the spacer on the fixture. The base portion of the spacing cylinders is adapted to the hexagon shape of the fixture, so that positive locking is achieved when this portion is applied to the fixture.
In order to improve the aesthetic result of a tooth reconstruction, other types of spacing members are also already known, see for example EP 0 291 103, EP 0 323 421 and ER 0 419 431. These spacing members also comprise an essentially cylinder-shaped spacer and a separate spacing screw.
In some cases it may be difficult to decide on the final type of spacing member which is to be used, or it may also be difficult to determine exactly the correct length of the spacer. In these cases, so-called healing spacers are used which are connected to the fixture temporarily while the final artificial crown is being made and tried out on the patient, but which are then replaced by the permanent spacer. Using a particular healing spacer improves the conditions for favorable incorporation of the mucous membrane, since the mucous membrane can be given a contour which is adapted to the contour of the permanent spacer. The healing spacers are similar to the standard spacers, but the sleeve and the screw are made in one piece. For thin reason, the healing spacers do not provide for positive locking to the upper part of the fixture.
Spacers which are made in this way, i.e. in which the screw and the actual spacing sleeve are in one piece, suffer from a number of disadvantages:
It is not possible to apply counterholding when the spacer is connected to the fixture, i.e. the entire tightening force is transmitted down to the fixture (the implant) which thereby runs a potential risk of becoming loose when the spacer is being tightened or loosened. PA1 On account of the large diameter of the screw head, a considerable torque is required in order to achieve proper tightening of the screw connection. PA1 Should the spacer become loose, the screw may be subjected to a rocking movement, which can lead to the spacer breaking. PA1 All spacers have had a circular symmetrical-shape. since the spacer cannot be positioned. PA1 It is not possible to use one material for the screw and another material for the actual spacer, although this may be desirable since the spacer must be made of a biocompatible material and the screw should be made of a material with high strength.
All these disadvantages can be overcome by using a spacing member which has a separate spacing screw, but this is at the expense of the components being more difficult to handle. Since the components are small, and since the space for handling them is also small, this represents a serious disadvantage in the case of those spacing members which are not made as one piece only. There is also a potential risk of dropping the small spacing screw and of the latter slipping down the patient's throat.
It is already known to arrange a spacing member with a separate screw in such a way that the screw and the spacer are made integral by means of the screw having been screwed through a threaded passage in the spacing sleeve, see EP 0 456 777. In this way it is possible to facilitate use of the spacing member and to reduce the risk of components being dropped. However, despite the fact that the spacing member has a separate spacing screw, it has the same disadvantages as regarding the risk of the tightening force being transmitted down to the fixture when the spacing member is fitted. The frictional force between the spacing sleeve 2 and the fixture 1 is in fact quite sufficient to be able to transmit the tightening force to the fixture.
Moreover, in the construction which is shown in EP 0 456 777, the spacing sleeve 2 is provided with an internal thread 23 at the far bottom of the continuous passage. This means that the threaded portion 34 of the spacing screw, after it has been screwed via the thread 23, comes to be situated completely outside the spacing sleeve 2. This makes it difficult to fit the spacing member to the fixture 1, since both the spacing sleeve and the screw have to be fitted simultaneously to the fixture. There is admittedly a free area (unthreaded part) 14 in the fixture which accommodates the threaded portion 34 of the spacing screw before the screw is screwed firmly into the threaded bore 13 of the fixture, but the majority of today's existing fixtures have a bore which is threaded right to the top, which makes it problematic to fit a spacing member, of the type which is shown in the European patent, in terms of getting the spacing screw into the correct position for threading-in, and at the same time the spacing sleeve cannot assume its final position in relation to the fixture. This position can be assumed only after the spacing screw has been screwed down a certain distance in the fixture.
A further disadvantage of the known construction is that the spacing screw, after it has been screwed via the thread 23 in the spacing sleeve, has its entire threaded portion 34 exposed, which increases the risk of damage to the thread upon handling, for example during transport.